OFDM multiple access (OFDMA) [Under Multiple Access]
Simply speaking OFDMA is how to use OFDM for serving multiple access/users. One way would be to group subcarriers. Each group is a subchannel. Then this subchannel can be allocated (dynamically) to a user.
Selection of subcarriers can be done in consecutive way (known as Adjacent Subcarrier Method or ASM) or in (seemingly) random way (known as Diversity Subcarrier Method or DSM). WiMAX mandates use DSM.
For ASM, we may choose subchannels (f1, f, f3), (f4, f5, f6), (f7, f8, f9).
For DSM, it may be (f1, f5, f7), (f2, f4, f8), (f3, f6, f9).
On top of it, Time Division Duplex (TDD) or Frequency Division Duplex (FDD) can be used for Uplink (UL) Downlink (DL) separation and allocation.
In addition to selection of subcarriers, further decision is to be made in terms of "Number of subcarriers" and "Subcarrier spacing" in relation to Bandwidth (frequency band spacing, say 2.5GHz +/- 5 MHz). One way is to keep "Number of subcarriers" same irrespective of Bandwidth. Another way is to keep "Subcarrier spacing" constant irrespective of Bandwidth. This method is preferred from hardware implementation point of view. It is known as Scalable OFDMA as with Bandwidth, number of subcarriers increase and so the data rate. Popular example of SOFDMA is WiMAX.
Typically range of frequencies, bandwidths, subcarrier spacings are set by standard/specification (like 802.16)/industry wide body (like WiMAX).
OFDMA References: In addition to Wikipedia page on OFDMA, check out OFDMA Paper from au-kbc.org.
© Copyright Samir Amberkar 2010
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